Dalton
Transactions
View Article Online
Open Access Article. Published on 11 November 2015. Downloaded on 15/06/2017 20:59:42.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
PAPER
Cite this: Dalton Trans., 2015, 44,
21099
View Journal | View Issue
The formation mechanism of iron oxide
nanoparticles within the microwave-assisted
solvothermal synthesis and its correlation with
the structural and magnetic properties
Z. Kozakova,* I. Kuritka, N. E. Kazantseva, V. Babayan, M. Pastorek, M. Machovsky,
P. Bazant and P. Saha
Magnetic nanoparticles based on Fe3O4 were prepared by a facile and rapid one-pot solvothermal
synthesis using FeCl3·6H2O as a source of iron ions, ethylene glycol as a solvent and NH4Ac, (NH4)2CO3,
NH4HCO3 or aqueous NH3 as precipitating and nucleating agents. In contrast to previous reports we
reduce the synthesis time to 30 minutes using a pressurized microwave reactor without the requirement
of further post-treatments such as calcination. Dramatically reduced synthesis time prevents particle
Received 9th September 2015,
Accepted 11th November 2015
DOI: 10.1039/c5dt03518j
www.rsc.org/dalton
growth via Ostwald ripening thus the obtained particles have dimensions in the range of 20 to 130 nm,
they are uniform in shape and exhibit magnetic properties with saturation magnetization ranging from 8
to 76 emu g−1. The suggested method allows simple particle size and crystallinity tuning resulting in
improved magnetic properties by changing the synthesis parameters, i.e. temperature and nucleating
agents. Moreover, efficiency of conversion of raw material into the product is almost 100%.
Introduction
Recently, the interest in preparation of nano-sized magnetic
particles has risen due to their potential utilization in many
areas of use. The most ambitious applications are in medicine
and pharmacy for magnetic resonance imaging, biomaterial
diagnostics, and drug delivery and in cancer treatment: magnetically mediated hyperthermia. Furthermore, they are used
in the preparation of functional magnetic materials, high
recording media, coloured pigments and ferrofluids.1–6 All of
these applications require defined nanoparticle features
together with high magnetization. Therefore, the development
of a synthetic method enabling the preparation of a tailormade product with a truly wide range of required properties
attracts great attention. Lots of methods have been introduced
for preparation of magnetite nanoparticles, by using both, dry
and wet chemistry. Thanks to the simplicity of the solvothermal method, it became one of the most popular among them.
It is based on the principle of heating a solution of a metallic
salt in a suitable solvent in the Teflon-lined stainless autoclave
in the presence of other substances, such as nucleating or
reducing agents and surfactants. Recently, several groups have
described a relatively simple preparation of well-crystallized
Centre of Polymer Systems, Tomas Bata University in Zlin, tr. Tomase Bati 5678,
760 01 Zlin, Czech Republic. E-mail: [email protected]
This journal is © The Royal Society of Chemistry 2015
magnetic nanoparticles where ferric chloride is the source of
iron cations and ethylene glycol serves both as a solvent and a
reducing agent. Moreover, other substances used in synthesis
were ethylenediamine serving as both, a coordinating agent
and a quasi-surfactant, urea or different ammonium salts used
as nucleating agents, and sodium acetate or other
chemicals.7–10 However, tardiness of all of these methods (synthesis time at least 12 hours) is their main disadvantage which
should be solved in order to improve the efficiency and applicability of solvothermal syntheses. Microwave heating, using
the transformation of electromagnetic energy to heat, seems to
be a suitable alternative to conventional heating leading to
acceleration of the synthetic procedure. Common microwave
reactors use radiation of frequency 2.45 GHz and wavelength
12.2 cm. Microwave synthesis is used in both organic and inorganic chemistry as a rapid method durable usually for
several minutes and therefore it is also energy saving. It is
possible to operate with a reflux system at atmospheric
pressure that provides preparation of particles of various sizes
and shapes, such as spheres, sheets, rods, tubes and others as
it was demonstrated recently.11–13 Several microwave-assisted
methods utilizing the reflux system have been used for synthesis of magnetic particles. However, these methods are relatively tedious and time-consuming as it turned out. As an
example, two hours are required for preparation of Fe3O4
nanoparticles by MW heating of the product obtained by
mixing the solutions of FeCl3 and FeSO4 under argon protec-
Dalton Trans., 2015, 44, 21099–21108 | 21099
View Article Online
Open Access Article. Published on 11 November 2015. Downloaded on 15/06/2017 20:59:42.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Paper
tion with the addition of aqueous NH3.14 In another case, magnetite and maghemite nanoparticles were synthesized by the
fast and simple method based on MW heating of a precursor
obtained from the mixture of FeCl3 and polyethylene glycol
(Mw = 20 000) in distilled water with the addition of hydrazine
hydrate; however, the as-obtained particles have relatively wide
size distribution and irregular shapes.15 According to our
experience, heating limited by boiling temperature, intense
mass flows accompanying the evaporation and subsequent
condensation of the solvent in external coolers and foaming of
the reaction mixture are strong limitations for these systems.
Although a pressurized system for solvothermal synthesis
offers relatively simple control over the morphology and particle size, examples of microwave pressurized reactors are
reported very rarely in this field. Caillot et al. used a microwave
reactor connected with an autoclave (RAMO system, Reacteur
Autoclave Micro Onde) for the synthesis of magnetic nanoparticles using FeCl2·4H2O and sodium ethoxide as the starting material.16 The resulting products were Fe2O3, Fe3O4 or
their mixtures depending on the concentration of sodium
ethoxide. Microwave-assisted syntheses using a pressurized
system are based on the exposure of reaction mixtures to MW
radiation in sealed vessels made of PTFE (Teflon) or even glass
in microwave ovens. Products of such synthesis are influenced
by the initial setting of synthetic parameters, namely the concentration of metallic salts and the type of solvent. On the
other hand, a programmable MW system with a precise
control of temperature and pressure may offer full control over
the synthesis conditions.17 In many cases, the temperature
increase is needed for initiation of nucleation in nanoparticle
synthesis. If conventional heating is used, the reaction mixture
is heated through the wall of the reaction vessel via conduction
and convection of heat, and thus the created temperature gradient causes non-uniform nucleation and growth conditions.
Microwave heating can solve this problem, since the energy is
directly transferred to the microwave absorbing materials and
the total volume of the reactant is therefore heated, while the
uniformity of heating is assured by the positioning of reaction
vessels on a rotating carousel thus overcoming the main drawback of multimode MW ovens.18 There are two MW heating
effects that determine the quality of the final substance:
thermal and non-thermal effects. The thermal effect is considered as a fast and effective heating of the reaction mixture
providing uniform nucleation and growth conditions and thus
leads to preparation of uniform nanoparticles with small sizes
and high crystallinity. Non-thermal effects include the creation
of hot spots and hot surfaces during the heating of solid
materials on the liquid–solid surfaces which support the
reduction of metal precursors, nucleation and formation of
metal clusters.11 These effects are still a matter of intensive
discussion and controversies in synthetic chemistry. Moreover,
the mechanisms of microwave effects playing the role in solvothermal synthesis of magnetic nanoparticles remain uncovered in the recent literature. The goal of this work is
threefold. First is to establish a simple, fast and highly
efficient method of magnetic nanoparticle synthesis with a
21100 | Dalton Trans., 2015, 44, 21099–21108
Dalton Transactions
view to the scientific and commercial uses. Towards this
purpose we decided to combine the simplicity of solvothermal
methods and the efficiency of microwave heating into a novel,
environmentally friendly method providing the product of fine
quality in high yields. Next, this work contributes to the elucidation of effects of microwave heating on the processes and
reactions that occur during the synthesis. Understanding the
correlation among the reaction mechanism, conditions and
properties of resulting particles enables fulfilment of the third
goal – development of a simple method with the possibility to
tailor the properties of the product.
Materials and methods
Solvothermal synthesis of magnetic nanoparticles
Nano- and submicro-sized Fe3O4 particles were prepared by a
simple solvothermal method in ethylene glycol solution with
the help of microwave irradiation. All reagents used for the
synthesis were purchased from Penta Ltd (Czech Republic) of
analytical grade and used without further purification. In a
standard experiment, 5 mmol of FeCl3·6H2O (1.352 g) was dissolved in 60 mL of ethylene glycol, followed by the addition of
50 mmol of ammonium acetate – NH4Ac (3.854 g). This
mixture was placed in a 100 mL Teflon reaction vessel (XP-1500
Plus), heated in a CEM Mars 5 microwave oven to the required
temperature (200 °C, 210 °C, 220 °C) and maintained at this
temperature for 30 minutes. After the reaction, the vessel was
cooled to room temperature and a black precipitate was collected with the help of a permanent magnet. Subsequently, the
as-obtained product was washed with distilled water and
ethanol for several times and dried naturally in air. Furthermore, in similar experiments, 25 mmol of (NH4)2CO3 (2.403 g),
50 mmol of NH4HCO3 (3.953 g) or 50 mmol of NH3 in the
form of aqueous solution (3.8 mL of 25% water solution) was
used as a nucleating agent instead of NH4Ac. Further experiments involved the addition of 2 to 4 mL of demineralized
water to the reaction system while the rest of parameters
remained unchanged. In all of the experiments, Fe(III) was
used as the only source of iron since there is no necessity to
work under Ar (or other inert atmosphere) to prevent Fe(II)
from oxidizing into Fe(III) and no need to work at a high pH to
obtain uniform particles. Moreover, the reducing agent used,
ethylene glycol, reduces Fe(III) to Fe(II) thus both oxidation
states are present in the reaction system.
Characterization methods
First of all, the macroscopic appearance of the prepared
powder was observed by naked eyes and by using a digital
microscope DVM 2500 (Leica Microsystems, Germany). The
structure of the final product was characterized by X-ray Diffraction (PANalytical X’Pert PRO) with Cu Kα1 radiation (λ =
1.540598 Å). Phase composition and crystallite sizes were
determined according to Rietveld analysis. Particle size and
morphology were preliminarily investigated with the help of
Scanning Electron Microscopy (VEGA\\LMU, Tescan). The
This journal is © The Royal Society of Chemistry 2015
View Article Online
Dalton Transactions
The X-ray diffraction (XRD) patterns of samples exemplified on
materials prepared with NH4Ac at 200, 210 and 220 °C for
30 minutes can be seen in Fig. 1. The diffraction pattern of the
material prepared at 220 °C is clearly attributed to the cubic
crystal structure of Fe3O4, however, the presence of peaks that
do not belong to the magnetite are observed in the patterns of
materials prepared at 200 and 210 °C, indicating creation of
other crystalline phases at lower synthesis temperatures. In
addition, although the diffraction pattern of the product of
synthesis proceeding at 200 °C includes broadened regions
mostly seen between 30 and 40° 2θ that indicate the presence
of amorphous portion and broad peaks characteristic of very
small particles, these features gradually disappear while the
synthesis temperature increases, and at 220 °C are even
absent. Therefore we can consider this system very sensitive
within the range of a few tens of degrees centigrade and
exploit the elevated temperature to accelerate the nucleation
and growth of the nanoparticles of the required crystalline
phase. The demonstrated development of sample crystallinity
with the temperature increment is also followed by the set of
the samples nucleated by other ammonium salts used in our
experiments; however, it is not shown here for the sake of
brevity. Calculation according to the Rietveld analysis showed
that the crystallite size of the magnetite phase depends on the
used nucleating agent and varies from 20 to 80 nm. Phase composition also differs with the manner of nucleation. While the
product prepared with aqueous ammonia is composed of pure
Fe3O4 without other crystalline impurities, the presence of minor
ferric compounds such as FeO(OH) and α-Fe2O3 is evident in
samples nucleated by ammonium carbonate and ammonium
bicarbonate and utilization of ammonium acetate leads to the
formation of the product with FeO(OH) in bigger portions.
It is well known that maghemite always accompanies magnetite in all materials due to the slow oxidation of magnetite
into maghemite by air oxygen and results in the formation of
the maghemite layer on the surface. In nanoscaled materials,
this effect becomes considerable due to the large surface area
and can be observed even in single crystals.19–21 Moreover, the
reactants and the mechanism (will be discussed later) of the
presented synthesis allows formation of maghemite and magnetite. Usually, the maghemite phase can be determined by
Mössbauer spectrometry.22,23 However, the differentiation
between the non-stoichiometric magnetite and magnetite–
maghemite mixture is considerably challenging and was even
claimed to be almost impossible.24,25 The same spinel structure
and almost identical lattice parameters make identification of
magnetite (Fe(II)Fe(III)2O4) and maghemite (γ-Fe(III)2O3) by the
XRD technique complicated. However, a deep detailed analysis
Fig. 1 XRD patterns of magnetic particles prepared with NH4Ac at
various temperatures for 30 minutes.
Fig. 2 Distinguishing between the magnetite and maghemite by
detailed analysis of the diffraction peaks localized in the angle range
56.5–57.5° 2θθ.
particle shape, organization and proportions were further
specified by Transmission Electron Microscopy (JEOL 1200,
JEOL). Image analysis was used for estimation of the particle
size distribution. Magnetic properties were measured by a
vibrating sample magnetometer (VSM 7400, Lake Shore).
Results and discussion
Open Access Article. Published on 11 November 2015. Downloaded on 15/06/2017 20:59:42.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Paper
Correlation of particle properties with reaction conditions
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 21099–21108 | 21101
View Article Online
Open Access Article. Published on 11 November 2015. Downloaded on 15/06/2017 20:59:42.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Paper
Dalton Transactions
of the (511) Bragg peak at the 2θ range 56.5–57.5° and the (440)
peak at the 2θ range 62–63.5° provides insight into this issue.26
Pure magnetite has the central position of the diffraction peak
at 57.0° while maghemite has this peak slightly shifted to
higher values, i.e. 57.3°. Under certain conditions, the observed
peaks can be analysed by deconvolution with the use of suitable
magnetite and maghemite standards. A detailed analysis of
XRD patterns obtained for our samples are shown in Fig. 2. It is
evident that all peaks are composed of more than one Gaussian
contribution which can be attributed to the presence of both
magnetite and maghemite phases in all samples. On the other
hand, the peak positions at X-axes can be slightly shifted
because the used diffractometer is not equipped with the Göbel
mirror. Without the use of high resolution XRD and pure magnetite and maghemite nanoparticulate standards, the deconvolution cannot be successfully performed and the ratio between
the phases cannot be exactly estimated. A scanning electron
microscopy image (SEM) of particles prepared with ammonium
acetate is shown in Fig. 3. The particles have a quasi-spherical
shape and occur in the form of clusters. Similarly, an SEM
study of all types of prepared products provides analogous
results, which can be influenced by the resolution of the used
method. The images of better resolution were obtained by transmission electron microscopy (TEM) (Fig. 4). Analyses of the
Fig. 3
SEM image of material prepared with NH4Ac for 30 minutes.
Fig. 4
TEM images of materials prepared at 220 °C for 30 minutes with (NH4)2CO3 (a), NH4HCO3 (b), aq. NH3 (c) and NH4Ac (d).
21102 | Dalton Trans., 2015, 44, 21099–21108
This journal is © The Royal Society of Chemistry 2015
View Article Online
Open Access Article. Published on 11 November 2015. Downloaded on 15/06/2017 20:59:42.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Dalton Transactions
TEM images show that the importance of a nucleation agent on
the particle size, shape and organization is evident. The use of
aqueous NH3 as well as NH4HCO3 provides a single-crystalline
material composed of uniform-shaped polyhedral nanoparticles
with narrow size distribution and dimension below 30 nm.
However, the use of (NH4)2CO3 and NH4Ac for nucleation lead
to the creation of clusters consisting of nano-sized grains and
ranging to several hundred nanometers. The hierarchical structure of clusters affects the magnetic properties of materials
obtained and therefore we further studied the reason for their
formation within the microwave-assisted solvothermal process.
The size of clusters obtained with the use of ammonium acetate
based on TEM analysis is 60 nm. Since the size of grains of the
magnetite phase calculated by Rietveld analysis is about 40 nm,
Fig. 5
Paper
these clusters seem to be formed by the binding of the small
crystallites of the non-magnetic phase together with the magnetite crystals, as can be seen in the TEM image at high magnification (Fig. 4d). While the ammonium carbonate is used,
particles with a diameter of about 120 nm are formed due to
the twinning of crystallites of magnetite with a diameter of
80 nm induced by the incorporation of crystalline impurities
(such as FeO(OH) and α-Fe2O3) produced during the synthesis.
Nanoparticle sizes are confirmed by calculations based on Rietveld analysis as described above.
Information on particle sizes obtained by TEM image analyses was further used for creation of histograms which give us
a conception of particle size distribution in the prepared
materials (Fig. 5).
Size distribution histograms of materials prepared at 220 °C for 30 minutes with various nucleating agents.
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 21099–21108 | 21103
View Article Online
Open Access Article. Published on 11 November 2015. Downloaded on 15/06/2017 20:59:42.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Paper
The magnetostatic properties of samples were investigated
by using a vibrating sample magnetometer (VSM) at room
temperature. Fig. 6 (left) demonstrates the magnetization
curves of the material prepared with ammonium acetate synthesized at 200, 210 and 220 °C. Saturation magnetization (Ms)
of nanoparticles obtained with NH4Ac varies from 13.8 to
67.7 emu g−1 with regard to the increase of temperature. The
increase of Ms can be explained on the one hand to the crystallinity improvement seen in Fig. 1, and on the other hand probably due to the phase transition at higher temperature which
led to formation of the material with a major magnetite
(maghemite) fraction. Similar results were obtained for the
samples nucleated by (NH4)2CO3 with saturation magnetization ranging from 8.4 to 46.2 emu g−1, as well as for NH4HCO3
with Ms in the range of 33.2 to 75.2 emu g−1 and, also for
aqueous NH3 with the highest saturation magnetization
among the forenamed materials that reaches 44.5 to
76.3 emu g−1. Lower values of Ms of the prepared materials
than that of the bulk magnetite (92–100 emu g−1)27 are
common for nanoparticulate systems and can be attributed to
the canted spins on the surfaces.28 On the other hand, the
maghemite phase present on the surface of nanoparticles
could also lead to the decrease of Ms. For a better understanding of effects influencing the magnetic properties of the prepared materials, the magnetization curves obtained with the
use of different nucleating agents were plotted together in
Fig. 6 (right). Obviously, particles prepared with aqueous
ammonia and ammonium bicarbonate have the highest saturation magnetization; particles prepared with ammonium carbonate have the lowest one. It is well known that coercivity is
Dalton Transactions
determined by effective magnetic anisotropy and thus depends
on the material composition as well as the grain size and
shape. Fig. 6 shows the rise of the coercivity depending on the
synthesis temperature. It can be attributed to the already mentioned presence of the magnetite phase in the sample at the
expense of other less magnetic phases such as hematite or
non-magnetic goethite and the growth of the particles to the
larger dimensions. However, despite variations of the particle
size with the use of different nucleating agents, the coercivity
of the materials prepared by different ways of nucleation is
very similar and reaches about 60 Oe. Nevertheless, the
material prepared with the use of ammonium carbonate is the
only exception with nearly zero value of coercivity. The
squeezed shape of the hysteresis loop (see the inset graph in
Fig. 6) indicates the pronounced influence of the cluster formation on the hysteresis and confirms the presence of two
magnetic phases that differ in magnetic properties.29
The mechanism of nanoparticle formation by MW-assisted
synthesis
As the starting point for the description of the growth of Fe3O4
nanoparticles, the two-stage growth model of nanoparticles in
supersaturated solutions can be adopted.30 In the first stage,
nucleation of primary nanocrystals occurs followed by their
aggregation into the secondary nanoparticles and, especially,
this second step has to be reconsidered for specific microwave
conditions. We used NH4Ac, (NH4)2CO3, NH4HCO3 and
aqueous NH3 as nucleating agents. NH4Ac is a weak-acid–
weak-base salt that can be hydrolysed at high temperature in
Fig. 6 Magnetization curves of materials prepared with NH4Ac at various temperatures for 30 minutes (left) and at 220 °C for 30 minutes with
various nucleating agents (right).
21104 | Dalton Trans., 2015, 44, 21099–21108
This journal is © The Royal Society of Chemistry 2015
View Article Online
Dalton Transactions
Paper
the presence of a trace amount of water coming from
FeCl3·6H2O as follows:7
Open Access Article. Published on 11 November 2015. Downloaded on 15/06/2017 20:59:42.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
NH4 Ac þ H2 O Ð HAc þ NH3 H2 O
ð1Þ
Although NH3·H2O is used in ref. 7, we do not expect the
formation of solid ammonium hydrates in the system and we
consider this formula to be an expression of the presence of
ammonia and water (NH3 + H2O) as it was most likely the
author’s original intention.
Similarly, (NH4)2CO3 and NH4HCO3 can be hydrolysed into
NH3:
ðNH4 Þ2 CO3 þ H2 O Ð 2NH3 þ 2H2 O þ CO2
ð2Þ
NH4 HCO3 þ H2 O Ð NH3 þ 2H2 O þ CO2
ð3Þ
NH3, either as the product of the above reactions or added
as aqueous solution yields hydroxide anions.
NH3 þ H2 O Ð NH4 þ þ OH
ð4Þ
In this case, Fe(III) salt was used as the precursor. Precipitation of its hydroxide in the form of colloidal solution proceeds as follows:
Fe3þ þ 3OH Ð FeðOHÞ3
ð5Þ
Next, formation of Fe2O3 particles may continue according
to:
2FeðOHÞ3 Ð Fe2 O3 þ 3H2 O
ð6Þ
Hence the maghemite phase can be formed in this way.
Under the presence of a mild reducing agent (e.g. ethylene
glycol), Fe(II) may appear in the system. Ethylene glycol can
undergo dehydration and the so-formed acetaldehyde31,32
reduces Fe(III) to Fe(II) and gives ferrous hydroxide in the form
of a green colloid:
2HOCH2 –CH2 OH Ð 2CH3 CHO þ 2H2 O
2CH3 CHO þ 2Fe3þ Ð CH3 CO–COCH3 þ 2Fe2þ þ 2Hþ
Fe2þ þ 2OH Ð FeðOHÞ2
ð7Þ
ð8Þ
ð9Þ
Most likely, both ferric and ferrous oxidation states of iron
coexist in the reaction mixture thus enabling magnetite
formation:
2FeðOHÞ3 þ FeðOHÞ2 Ð Fe3 O4 þ 4H2 O
ð10Þ
A minor phase, goethite, was observed and its presence in
the prepared materials can be explained by partial
dehydration:
FeðOHÞ3 Ð FeOðOHÞ þ H2 O
ð11Þ
or by the following equation in the presence of oxygen:
4FeðOHÞ2 þ O2 Ð 4FeOðOHÞ þ 2H2 O
ð12Þ
The first step of nucleation starts with the first appearance
of heterogeneity in the system similarly, as in common solvothermal methods. Hu et al.7 proposed the mechanism with the
This journal is © The Royal Society of Chemistry 2015
key role of NH3·H2O. Aqueous ammonia evaporates and forms
gaseous bubbles that provide heterogeneous nucleation
centers for newly-formed nanoparticles, which can then aggregate around the gas–liquid interface. In this case, we used
various precipitation agents and all of them lead to nanocrystalline products in the same reaction time; however, we did
not obtain any evidence of hollow structures. Moreover, other
authors used ammonia-less techniques with sodium acetate
and obtained similar results, albeit this was achieved without
MW assistance.30,33 Lou et al. in their review34 consider a gas
templating mechanism that uses soluble gases especially that
originated from the decomposition of organic molecules such
as urea, as highly speculative. Therefore we propose that the
reaction starts with the formation of colloid according to the
eqn (5) and (9) rather than with the formation of nanobubbles.
In the second step, it is generally accepted for conventional
solvothermal methods that the freshly formed nanocrystals are
unstable due to high surface energy so they tend to aggregate.
The driving force for this aggregation is the pursuit of reducing the surface energy by both attachment among the
primary nanocrystals and their rotation caused by various
interactions including the Brownian motion or short-range
interactions.30,35 In our case of MW heating, the solvent at
high temperature (above 200 °C) becomes less absorbing due
to the decrease of dielectric constant and the loss factor thus
the solvent becomes virtually more transparent for microwaves,
which means that the ionic mechanism of microwave absorption may be prevailing over the dipoles.12 After the solid particles occur in the solution, another mechanism becomes
active due to particle surface polarization that might selectively
influence their growth. Moreover, as the heat is generated at
the surface of nanoparticles, the local temperature gradient
can contribute to the hydroxide-to-oxide transformation
described in eqn (6) and (10). Once the oxide particle reaches
the critical size corresponding to the single-domain state,
another mechanism of MW absorption is assumed to take the
main role. Microwave energy is transformed into heat by magnetic moment rotation and after some time in bigger multidomain particles also by the magnetic domain wall motion.
This sequence of several mechanisms can explain the sudden
formation of particles in the reaction mixture as well as the
very short synthesis time. The time required for the formation
of nanoparticles with magnetic properties exceeds 12 hours for
common solvothermal synthesis. Assuming simply just the
fast heating of the reaction mixture and no non-thermal
effects supporting the transformation of the raw material into
the product we reduced the synthesis time firstly to
20 minutes. However, the time of microwave treatment was not
sufficient for the precipitate formation and only opaque
yellow colloid was obtained with no magnetically separable
fraction. Prolongation of the exposure to microwaves to
30 minutes enables formation of a black precipitate and,
moreover, the conversion of the starting material into the
product runs with almost 100% efficiency. This fact refers to
the sudden formation of the final product, and the relatively
narrow particle size distribution and supports the concept of
Dalton Trans., 2015, 44, 21099–21108 | 21105
View Article Online
Open Access Article. Published on 11 November 2015. Downloaded on 15/06/2017 20:59:42.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Paper
Dalton Transactions
sequential action of MW absorption mechanisms as a nonthermal effect.
The role of solvent has to be reconsidered in comparison
with the conventional processes as well. Ethylene glycol is an
exceptionally good microwave absorbing solvent at 2.45 GHz
(tan δ = 1.350)36 which assures very fast increase of temperature at the beginning of the synthesis together with fast
achievement of quasi-stable reaction conditions in the pressurized reaction vessel. On the other hand, it is not directly
involved in the chemistry of ferric hydroxide or oxide formation. EG can disproportionate according to eqn (7) but the
equilibrium must be shifted to the left side of the reaction,
however the acetaldehyde enters the redox reaction (8) and
yields Fe2+ cations. Liberation of one H2O molecule per each
Fe2+ cation occurs according to eqn (7) and (8). A stoichiometric amount of water is delivered to the reaction system with
FeCl3·6H2O. Water plays a crucial role in cation solvation,
nucleation agent hydrolysis, see eqn (1)–(4), and in the formation of ferric and ferrous hydroxides, eqn (5) and (9).
Hence, water is not present in excess but in stoichiometric
amount as one of the reactants. Moreover, it has the highest
dielectric constant among common solvents although its loss
factor (tan δ = 0.123)36 is approximately eleven times lower
than that of EG.
In contrast to fast nucleation in aqueous solutions, aggregation in ethylene glycol is kinetically slower due to the fewer
hydroxyl groups on the particle surface and higher viscosity,
which allows adequate rotation of nanocrystals to form a lowenergy configuration of interface and perfectly organized
assemblies. Subsequently, these aggregates further crystallize
and form compact crystals that exhibit features of singlecrystal particles.9 As it is possible to see in Fig. 3, the particles
obtained with the use of aqueous NH3 are single crystals with
a polyhedral shape, the second most perfectly developed crystallites were observed for NH4HCO3 then followed by less developed spherical multi-grains obtained by the use of (NH4)2CO3,
whereas nucleation by NH4Ac led to the biggest polycrystalline
particles with a spherical shape. The same trend was observed
for phase composition of the prepared materials. As can be
seen in Table 1, the use of aqueous ammonia yielded a single
phase material while NH4Ac gave the highest fraction of
goethite in the product. The other nucleation agents
NH4HCO3 and (NH4)2CO3 gave moderate results. The observed
trend of perfection of crystalline structure development correlates well with the virtually hidden presence of water in nucle-
Table 1
ating agents. Hydrolysis of NH4Ac does not provide any extra
water molecule per each NH3 in eqn (1). Hydrolysis of
(NH4)2CO3 yields one extra H2O molecule per two NH3 and
hydrolysis of NH4HCO3 yields two extra water molecules per
each NH3, see eqn (2) and (3). About 2.5 mL of water gets to
the reaction system with the addition of aqueous ammonia
(25% aqueous solution), which means approximately 3 molecules of H2O per each NH3. Since the synthesis conditions
such as temperature and duration are the same in all four
cases, the influence of composition of the reaction mixture on
the particle shape and organization was further investigated. A
similar system was described by Cao et al.35 and denominated
as the (EG)–H2O system. By the addition of different amounts
of water (1–3 mL) into the reaction system, Fe3O4 polyhedral
particles with different sizes are formed whereas the samples
prepared without water addition led to the formation of the
product that consisted of spherical particles assembled by
smaller particles. If water is added to the reaction system, the
coordinated EG molecules will be substituted by water molecules since the coordination of water molecules to metal ions
is stronger than that of EG molecules. By selecting the amount
of water to be added, the particle size can be influenced in
agreement with Cao et al. We considered the role of addition
of deionized water as one of the most important factors in the
control of particle size and size distribution, too. For this
purpose we also performed experiments with the addition of
water into the reaction mixture. According to the TEM images
shown above, the sample nucleated by NH4HCO3 was chosen
due to the narrow size distribution and shape uniformity of
particles. Apparently from the TEM image seen in Fig. 7,
addition of small amount of demineralized water causes
reduction of particle size from 30 to 10 nm, even if the crystalline composition and design of the as-prepared particles
remain unchanged and they also possess a single-crystalline
character with a polyhedral shape. The size of particles
dropped to the value under which the particles are no longer
in a single-domain state and we obtained superparamagnetic
particles. This magnetic behaviour is desired for many applications, however, in many cases and also in this case, this transition also led to the decrease of Ms value (Fig. 7 (right)). Next
to the synthetic conditions that can be directly controlled such
as the synthetic temperature and composition of the reaction
mixture, pressure in reaction vessels can be influenced only
indirectly, through setting the level of the vessel filling, the
type of solvent and temperature (reaction temperature below
Variations of properties of materials prepared with different nucleating agents
Type of
nucleating agent
NH3 to H2O
molecules ratio
Crystalline
impurities
Fe3O4 crystallite
size by XRD (nm)
Particle size
by TEM (nm)
Ms
(emu g−1)
Hc (Oe)
(NH4)2CO3
NH4HCO3
NH3 aq.
NH4Ac
1 : 0.5
1:1
1:3
1:0
Minor Fe2O3·H2O, Fe2O3
Minor Fe2O3·H2O
—
Minor α-FeO(OH)
84
32
17
42
130
40
20
60
46
75
76
68
2
65
61
67
21106 | Dalton Trans., 2015, 44, 21099–21108
This journal is © The Royal Society of Chemistry 2015
View Article Online
Open Access Article. Published on 11 November 2015. Downloaded on 15/06/2017 20:59:42.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Dalton Transactions
Paper
Fig. 7 TEM image of material prepared with NH4HCO3 with the addition of water (left) and magnetization curves of the same material with and
without the water addition (right).
or above the boiling point of solvent) and also by the type of
used reactant. Since the used solvent and the level of vessel
filling was almost the same in our experiments, the pressure
in reaction vessels during the synthesis depended on the used
nucleation agents and varied from 900 to 3800 kPa. When
aqueous ammonia and ammonium acetate were used within
the synthesis, the lowest pressure (800–1000 kPa) was
obtained. On the other hand, the decomposition of
ammonium carbonate was accompanied with increasing the
pressure up to 3800 kPa due to the significant change of mole
ratio of the reaction (see eqn (2)). The use of ammonium bicarbonate leads to a medium value of pressure about 2000 kPa,
since a smaller change of mole ratio of reaction accompanied
the decomposition reaction.
The importance of the use of a pressurized reactor lies in
the possibility to superheat the solvent by which the synthesis
can be performed above the boiling point. Moreover, above the
boiling point, ethylene glycol becomes virtually transparent for
microwaves thus the reaction mixture behaves similarly as
ionic liquids that consisted entirely of ions and the ionic effect
becomes the prevailing reaction mechanism. The ionic effect
is considered to be much stronger than the dipolar rotation
mechanism and therefore the conversion of the raw material
into the solid product in the reaction system can be significantly enhanced. Moreover, the use of pressurized microwave
reactors with sealed vessels ensure a limited amount of oxygen
from the atmosphere to be involved in the reaction system
thus preventing the complete oxidation of Fe(II) into the Fe(III)
cations that lead to the formation of undesired by-products
such as α-Fe2O3.
This journal is © The Royal Society of Chemistry 2015
Conclusions
A simple and effective method for preparation of magnetic
material based on Fe3O4 in nano- and submicron-dimension
has been proposed. Efficiency of this method is to provide the
transformation of a common solvothermal method using conventional heating into a microwave-assisted method. The
change of heating method causes considerable acceleration of
the reaction due to the direct heating of the absorbing material
and non-thermal effects supporting the transformation of raw
material into the final product. Thanks to the use of a pressurized system enabling the control of the synthesis temperature
and time as well as indirectly total pressure in the reaction
vessel, materials with various properties that reflect the
requirements of the specific applications were prepared. Particles ranging from 20 to more than 100 nm based on singlecrystals or crystalline assemblies were prepared by selecting
the type of nucleating agent.
It is established that an appropriate composition of the
starting reaction mixture leads to the formation of the pure
one-phase or multiphase materials. Magnetic properties of
materials obtained by the proposed microwave-assisted
method can be tailored as a consequence of the changes in
the phase composition, crystalline structure, particles size,
shape and their self-organization into clusters. Besides the
change of reaction mixture by the variation of nucleating
agents, the reaction medium represented by ethylene glycol
can be modified by the addition of demineralized water. A
small portion of demineralized water in the reaction medium
results in the reduction of nanoparticle sizes to one-third of
Dalton Trans., 2015, 44, 21099–21108 | 21107
View Article Online
Open Access Article. Published on 11 November 2015. Downloaded on 15/06/2017 20:59:42.
This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
Paper
the original size that is significantly reflected in the magnetic
properties and can lead to the formation of nanoparticles in
superparamagnetic and ferrimagnetic states. Even the addition
of minor amounts of demineralized water to the reaction
system can serve as the tool for tailoring the magnetic properties of the final product and makes this microwave-assisted
solvothermal method more flexible to the current requirements. Consequently, simplicity, low cost and variability and
reproducibility of this method make it a suitable candidate for
the routine preparation of ferrimagnetic and superparamagnetic materials in nano-dimensions.
Acknowledgements
This work was supported by the Ministry of Education, Youth
and Sports of the Czech Republic – Program NPU I (LO1504).
Notes and references
1 A. K. Gupta and M. Gupta, Biomaterials, 2005, 26, 3995–4021.
2 D. K. Kim, Y. Zhang, J. Kehr, T. Klason, B. Bjelke and
M. Muhammed, J. Magn. Magn. Mater., 2001, 225, 256–261.
3 A. S. Lubbe, C. Alexiou and C. Bergemann, J. Surg. Res.,
2001, 95, 200–206.
4 S. W. Cao, Y. J. Zhu, M. Y. Ma, L. Li and L. Zhang, J. Phys.
Chem. C, 2008, 112, 1851–1856.
5 J. Jestin, F. Cousin, I. Dubois, C. Menager, R. Schweins,
J. Oberdisse and F. Boue, Adv. Mater., 2008, 20, 2533–2540.
6 C. Scherer and A. M. F. Neto, Braz. J. Phys., 2005, 35, 718–727.
7 P. Hu, L. J. Yu, A. H. Zuo, C. Y. Guo and F. L. Yuan, J. Phys.
Chem. C, 2009, 113, 900–906.
8 X. Y. Chen, Z. J. Zhang, X. X. Li and C. W. Shi, Chem. Phys.
Lett., 2006, 422, 294–298.
9 L. P. Zhu, H. M. Xiao, W. D. Zhang, G. Yang and S. Y. Fu,
Cryst. Growth Des., 2008, 8, 957–963.
10 H. Deng, X. L. Li, Q. Peng, X. Wang, J. P. Chen and Y. D. Li,
Angew. Chem., Int. Ed., 2005, 44, 2782–2785.
11 M. Tsuji, M. Hashimoto, Y. Nishizawa, M. Kubokawa and
T. Tsuji, Chem. – Eur. J., 2005, 11, 440–452.
12 A. Loupy, Microwaves in organic synthesis – a review, Wiley
VHC, Welhein, 2002.
13 P. Lidstrom, J. Tierney, B. Wathey and J. Westman, Tetrahedron, 2001, 57, 9225–9283.
14 R. Y. Hong, T. T. Pan and H. Z. Li, J. Magn. Magn. Mater.,
2006, 303, 60–68.
21108 | Dalton Trans., 2015, 44, 21099–21108
Dalton Transactions
15 W. W. Wang, Y. J. Zhu and M. L. Ruan, J. Nanopart. Res.,
2007, 9, 419–426.
16 T. Caillot, D. Aymes, D. Stuerga, N. Viart and G. Pourroy,
J. Mater. Sci., 2002, 37, 5153–5158.
17 Z. Kozakova, P. Bazant, M. Machovsky, V. Babayan and
I. Kuritka, Acta Phys. Pol., A, 2010, 118, 948–949.
18 J. A. Gerbec, D. Magana, A. Washington and G. F. Strouse,
J. Am. Chem. Soc., 2005, 127, 15791–15800.
19 A. P. Grosvenor, B. A. Kobe and N. S. McIntyre, Surf. Sci.,
2004, 565, 151–162.
20 P. Guardia, A. Labarta and X. Batlle, J. Phys. Chem. C, 2011,
115, 390–396.
21 J. Tang, M. Myers, K. A. Bosnick and L. E. Brus, J. Phys.
Chem. B, 2003, 107, 7501–7506.
22 A. G. Roca, J. F. Marco, M. D. Morales and C. J. Serna,
J. Phys. Chem. C, 2007, 111, 18577–18584.
23 A. C. Doriguetto, N. G. Fernandes, A. I. C. Persiano,
E. Nunes, J. M. Greneche and J. D. Fabris, Phys. Chem.
Miner., 2003, 30, 249–255.
24 R. E. Vandenberghe, C. A. Barrero, G. M. da Costa, E. Van
San and E. De Grave, Hyperfine Interact., 2000, 126,
247–259.
25 C. A. Gorski and M. M. Scherer, Environ. Sci. Technol., 2009,
43, 3675–3680.
26 W. Kim, C. Y. Suh, S. W. Cho, K. M. Roh, H. Kwon, K. Song
and I. J. Shon, Talanta, 2012, 94, 348–352.
27 R. M. Cornell and U. Schwertmann, The iron oxides: structure, properties, reactions occurences, and uses, Wiley-VCH,
Weinheim, 2003.
28 M. P. Morales, S. Veintemillas-Verdaguer, M. I. Montero,
C. J. Serna, A. Roig, L. Casas, B. Martinez and
F. Sandiumenge, Chem. Mater., 1999, 11, 3058–3064.
29 R. Skomski, J. Phys.: Condens. Matter, 2003, 15, R841–R896.
30 S. H. Xuan, Y. X. J. Wang, J. C. Yu and K. C. F. Leung,
Chem. Mater., 2009, 21, 5079–5087.
31 S. E. Skrabalak, B. J. Wiley, M. Kim, E. V. Formo and
Y. N. Xia, Nano Lett., 2008, 8, 2077–2081.
32 W. B. Smith, Tetrahedron, 2002, 58, 2091–2094.
33 A. Yan, X. H. Liu, G. Qiu, N. Zhang, R. R. Shi, R. Yi,
M. Tang and R. C. Che, Solid State Commun., 2007, 144,
315–318.
34 X. W. Lou, L. A. Archer and Z. C. Yang, Adv. Mater., 2008,
20, 3987–4019.
35 S. W. Cao, Y. J. Zhu and J. Chang, New J. Chem., 2008, 32,
1526–1530.
36 N. E. Leadbeater, Microwave heating as a tool for sustainable
chemistry, CRC Press, Boca Raton, 2012.
This journal is © The Royal Society of Chemistry 2015